Signs of underground ocean found on Saturn moon

By Elizabeth Landau, CNN

Updated 12:17 PM ET, Fri April 4, 2014

Saturn and its moons18 photos

NASA's Cassini spacecraft has spotted mysterious reddish streaks on the surface of Saturn's icy moon Tethys. The red streaks are only a few miles wide but several hundred miles long. The images were taken in April. Scientists aren't sure what's causing the streaks.

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Saturn and its moons18 photos

This mosaic of Saturn's moon Mimas was created from images taken by Cassini in February 2010. A recent study indicates the moon may contain a liquid water ocean.

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Cassini glided high above Saturn in October 2013 to capture this 36-image mosaic of the ringed planet. The colors of the planet appear natural, just as the human eye would see them.

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Plumes of water ice and vapor shoot up from the surface of Saturn's moon Enceladus in this two-image mosaic taken by Cassini in November 2009. Analysis by NASA scientists indicated that water can reach the Saturnian moon's surface.

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Saturn and its moons18 photos

A small, bright blip can be seen on the outermost edge of Saturn's rings in this image taken in April 2013. The bump in the smooth ring structure is an icy object that could provide clues to how Saturn's moons formed.

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This false-color image of Saturn's north polar storm looks like a giant red rose surrounded by green foliage. Measurements indicate the storm's eye is a staggering 1,250 miles across with cloud swirling as fast as 330 mph.

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Saturn's rings cast a narrow shadow on its surface in this image taken in August 2009.

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The scars of time and space mark the surface of Saturn's moon Enceladus. Saturn has at least 62 moons in its orbit.

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The Saturn-facing side of Enceladus is illuminated by light bouncing off the planet. Plumes of water ice can be seen streaming off the moon's southern pole.

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Saturn's moon Rhea is seen from approximately 174,181 miles away in this March 2013 image.

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Rhea's surface is pockmarked with craters from billions of years of impacts. The moon is Saturn's second-largest, with a diameter of 949 miles.

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Saturn's largest moon, Titan, has a diameter of 3,200 miles. It looks like a fuzzy orange ball because of its atmosphere.

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Though it's the largest moon orbiting Saturn, Titan is dwarfed by Saturn itself.

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Saturn has a small moon called Dione orbiting about 234,000 miles away. That's about the same distance Earth is from its moon.

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The surface of Dione is seen in this May 2012 image.

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Tethys, top left, is dwarfed by Saturn as it orbits the planet, though scientists think the moon is much larger than Saturn's ring system.

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The Odysseus Crater spans 280 miles across the northern hemisphere of Tethys.

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Saturn and its moons18 photos

The small bright dot seen in the bottom right is not another Saturn moon. It's Earth. The distance between Saturn and our planet is constantly changing because both are constantly in motion. When they are closest together during their orbits, Saturn is 746 million miles away from Earth. At its farthest, they are just over a billion miles apart. See Cassini's top 10 discoveries about Saturn

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Story highlights

Enceladus is a small moon of Saturn

New data suggest a subsurface ocean under its south pole

Gravity experiment inferred that there must be liquid water there

Miles of ice likely cover a subsurface ocean that itself is 5 to 10 miles deep

An ocean at least as large as Lake Superior lies below a thick layer of ice on a moon of Saturn, new data from NASA's Cassini spacecraft suggests.

The results, published in the journal Science, support earlier signs that this small moon has liquid water. That means Saturn's sixth-largest moon could have been -- or could now be -- hospitable to life.

This discovery puts Enceladus in an exclusive club of extraterrestrial worlds in the solar system that appear to have a subsurface water ocean. The others are Titan, another moon of Saturn, and Europa, a moon of Jupiter. Callisto and Ganymede, also moons of Jupiter, may also have oceans under ice.

"As far as whether one should go first to Europa or Enceladus, I look at this as a kind of a cornucopia of habitable environments in the outer solar system," study co-author Jonathan Lunine of Cornell University said in a press conference Wednesday.

What we think is there

The north pole of Enceladus has a thickness of ice around 30 miles, with solid rock beneath it. But at the south pole, there may be only 18 to 24 miles of ice covering a subsurface ocean that itself is 5 to 10 miles deep.

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This ocean appears to be a "lens-shaped reservoir" that's deepest at the south pole and thinner further afield. The ocean appears to sit on top of a rocky core and "may extend halfway or more towards the equator in every direction," said study co-author David Stevenson, professor of planetary science at California Institute of Technology, in a press conference.

Also noteworthy is that the north pole is heavily cratered, while the ice on the south pole is smoother, meaning it has been resurfaced and softened.

Measurements from the new study and previous Cassini data suggest the moon's south pole has an ice layer on top of water on top of silicate rock. If that's true, water circulating through the rock would pick up elements such as phosphorous, sulfur, potassium and sodium into the liquid. These elements are essential for making molecules that life needs, Lunine said.

With respect to the ocean being on top of rock, the gravity data from Enceladus "makes the base of the ocean very much like the base of our own ocean on the Earth," Lunine said at the press conference.

Enceladus is a small moon, with a diameter of only about 310 miles. In 2005, Cassini found that fractures called "tiger stripes" in the south polar region emit water vapor jets rich in salt, lead author Luciano Iess of Sapienza Università di Roma in Rome, Italy, said in a press conference Wednesday.

Cassini has also detected organic molecules, which can come from biological or nonbiological sources, near the tiger stripes and in dust grains in the region.

No one has actually gone to Enceladus and seen this ocean under the thick ice layer. But scientists have strong evidence that it is there, based on gravity measurements made with Cassini.

When the spacecraft flies by Enceladus, the gravity of the moon changes the probe's speed. Scientists can measure those changes back on Earth by detecting fluctuations in the frequency of the signal that Cassini sends back. If the spacecraft's speed doesn't change, the frequency would remain the same.

But depending on where Cassini flies, the frequency of the signal changes in particular ways. This allows scientists to learn more about the moon's subsurface features.

Scientists had already known about the presence of a depression or "dimple" at the south pole of Enceladus, "as if there is some amount of, effectively, missing material," Lunine said. But in this new study, the change in gravity associated with the depression was not as dramatic as expected.

"There must be some higher density layer that's underneath the ice, and that has to be a liquid water ocean, because there's really nothing else plausible that can explain that offset to what we would expect for the gravity signature of this depression," he said.

It is possible that this ocean has been sending out the plumes that Cassini first observed in 2005, but this has not been verified.

How can we look for signs of life there?

Sending a probe to Enceladus with a drill wouldn't be entirely practical because the liquid water is so far under the ice that it would be hard to access, Lunine said.

Instead, a spacecraft with instruments more sophisticated than Cassini could fly through the plumes of material being ejected from fractures in the ice. Cassini has already detected the water vapor and organic molecules in these plumes, using a device called a mass spectrometer.

But a more advanced, more sensitive version of the same instrument could better test for the "menu of molecules" associated with an advanced biological system, Lunine said.

Detecting these molecules could be "the smoking gun for whether in fact there is life down there or not," he said.